In the field of automated control, two types of general systems are common. First, open-loop systems can be used to move a mechanical element with a predetermined action or command. These open-loop systems do not use feedback, and hence do not inherently compensate for how well the mechanical element responds to the command. Second, closed-loop systems are used to implement feedback, and thereby adjust drive of the mechanical element so that it accurately implements the command. One example of a closed-loop system is steering of a car; a heading is chosen for the car and the steering wheel is turned until, using his eyesight, a driver determines that the car has the approximate heading desired. If the driver turns the car too much, and it moves past the desired heading, eyesight informs the driver that the steering wheel must be turned back in the opposite direction to correct the car's heading. Thus, the driver may not know precisely the car's direction in degrees, but the driver uses feedback provided by his own eyes to steer the car in the desired direction. Closed-loop systems are generally more accurate than open loop systems, because they inherently compensate for mechanical inaccuracies.
Closed-loop systems are commonly used to control aeronautical surfaces, such as missile fins. Typically this is done by sensing the direction, or angle at which the fin is inclined, comparing that angle with the angle the fin is supposed to have, and producing an error signal as a result. This error signal is used to further steer the fin.
Although commonly used in some missile guidance systems, position feedback has sometimes proven inadequate. For example, missiles sometimes have four fins, grouped into two symmetrical pairs that form the shape of a cross around the missile. That is to say, some missiles use two pairs of fins, each fin oriented 90.degree. apart. Oftentimes, the motion of the missile and its own body effectively impose differing forces upon each fin of the missile. Even though one symmetric pair of fins may be horizontal, for example, each fin in that pair might experience different forces. Consequently, position feedback, which is generally used to commonly drive one symmetric pair of missile fins at the same time, may be imprecise, as one of the two fins in the pair can affect the missile's path more than the other fin in the pair. This can cause unwanted roll, yaw or pitch.
In the past, some attempts have also been made to investigate torque-controlled actuators, that is, an actuator which tells its mechanical element how hard and fast to move, rather than by how much. However, in the context of missile guidance systems, it is difficult with such designs to control the excursion of the missile fin past predetermined points. It is usually desired to rotate the missile fin only within a .+-.30.degree. range, to prevent excessive missile fin rotation from causing the missile to stall or otherwise move counter-productively. Torque-control has typically not been used for active guidance of a missile, and typically has been used only in limited circumstances. For example, one known design measures differential pressures on either side of a fin surface, to feather the corresponding symmetric fin pair to a null position during the booster stage of a missile or rocket. However, position feedback is then relied upon during later, active guidance of the missile or rocket.
There has existed a definite need for a torque feedback actuator that can be used for active guidance of an aeronautical surface. What is needed is a torque feedback system that can be used to modify an input torque command, so that the surface does not cause unwanted yaw, pitch and roll. Further, there has existed a need for an actuator that electronically prevents excessive excursion of the surface outside of a predetermined range, and preferably does so in the context of torque feedback. The present invention solves these needs and provides further, related advantages.